1. Field of the Invention
This invention generally relates to semiconductor fabrication and, more particularly, to a method for forming a relaxed silicon germanium film with high germanium content on a silicon substrate.
2. Description of the Related Art
In enhanced mobility MOSFET device applications, thick relaxed silicon (Si) germanium (Ge) layers have been used as substrates for thin strained Si layers, to increase carrier mobility for both NMOS and PMOS devices. Compared with bulk Si devices, enhancements in electron mobility of up to 70%, for devices with a channel length of less than 70 nanometers (nm), have been reported. Enhancements of up to 40% in high-field hole mobility for long-channel devices have also been found.
Conventionally, a high quality relaxed Si1xe2x88x92xGex buffer layer is formed by growing a several microns (xcexcm) thick compositionally graded layer, where Si1xe2x88x92xGex represents a silicon germanium film with a varying content of Ge. However, the density of threading dislocations is still high (typically  greater than 106/cm2). In addition, the integration of Si1xe2x88x92xGex film several microns thick into device fabrication has not been practical.
Alternative methods have been developed to efficiently relax strained SiGe layers on Si. The methods involving the implantation of hydrogen for the relaxation of strained SiGe layers have all utilized ionized atomic hydrogen (H+). However, this implantation process is expensive due to the long time required. Helium implantation followed by an anneal step has also been explored to promote relaxation in SiGe films.
It would be advantageous if singly ionized molecular hydrogen (H2+) could be used in the relaxation of SiGe films to reduce the process time and cost, since this implantation process can be done at double the energy and half the current.
It would be advantageous if boron, He, Si, or other species could be co-implanted with singly ionized molecular hydrogen (H2+), as they have been shown to be effective for silicon on insulator (SOI) fabrication.
It would be advantageous if the implantation of H2+ alone, or with a species such as boron, could be used for relaxing strained SiGe films deposited epitaxially on Si substrates.
The present invention method produces a thick (100-500 nm) relaxed, smooth SiGe film with high Ge content of greater than 20-30% as a substrate layer for a tensile strained Si film to be used for high speed MOSFET applications. As mentioned above, atomic hydrogen (H+) implantation has been shown to be effective for producing such films, however, this implantation process is very expensive due to the long process times required. The use of singly ionized molecular hydrogen (H2+) reduces the time and cost of the process, since the implant can be done at double the energy and half the current. Further, the H2+ can be implanted alone, or with boron, He, Si, or other species for the purpose of relaxing strained SiGe films deposited epitaxially on Si substrates.
Accordingly, a method is provided for forming a relaxed silicon germanium layer with a high germanium content on a silicon substrate. The method comprises: depositing a single-crystal silicon (Si) buffer layer overlying the silicon substrate; depositing a layer of single-crystal silicon germanium (Si1xe2x88x92xGex) overlying the Si buffer layer having a thickness of 1000 to 5000 xc3x85; implanting the Si1xe2x88x92xGex layer with ionized molecular hydrogen (H2+) a projected range of approximately 100 to 300 xc3x85 into the underlying Si buffer layer; optionally, implanting the Si1xe2x88x92xGex layer with a species selected such as boron, helium (He), or Si; annealing; and, in response to the annealing, converting the Si1xe2x88x92xGex layer to a relaxed Si1xe2x88x92xGex layer. Optionally, after annealing, an additional layer of single-crystal Si1xe2x88x92xGex having a thickness of greater than 1000 xc3x85 can be deposited overlying the relaxed layer of Si1xe2x88x92xGex.
Some aspects of the method include depositing a layer of Si1xe2x88x92xGex, where x is greater than 0.2. Alternately, a layer of graded Si1xe2x88x92xGex can be deposited, where x varies in the range from 0.03 to 0.5. Further, the Si1xe2x88x92xGex deposition process includes epitaxially growing the layer of Si1xe2x88x92xGex at a temperature in the range of 400 to 600 degrees C. to form a strained layer of Si1xe2x88x92xGex film, having a lattice structure that matches the underlying single-crystal Si buffer layer lattice structure.
The annealing process is conducted at a temperature in the range of 650 to 1000 degrees C. for a period of time in the range of 0.1 to 30 minutes. Alternately, the annealing process comprises: a low-temperature annealing at a temperature of approximately 250 degrees C. for a period of approximately 10 minutes; and, a high-temperature annealing at a temperature in the range of 650 to 1000 degrees C. for a period of time in the range of 0.1 to 30 minutes.
Additional details of the above-described method are provided below.